Main operated electric fence energizer

ABSTRACT

An electric fence energizer is operated by an alternating current and has two storage capacitors (C 1 , C 2 ), which are charged by a charging circuit (7) to a high voltage. The storage capacitors (C 1 , C 2 ) are discharged through separate primary windings (L 1 , L 2 ) in a transformer (T) and the secondary winding (L 3 ) of the transformer is connected to the electric fence. The discharging processes are controlled by separate discharging circuits (R 1 , T y1  ; T y3  and T y3  respectively), so that for light loads only one of the storage capacitors (C 1 ) is discharged, and for heavy loads also the other one (C 2 ) is discharged starting a short time after the start of the discharging of the first one and during the same discharge cycle. Sense circuits (L 2 , R 8 , R 9 , C 3 , R 10 , R 11 , R 12 , T 4 , R 13 ) provide signals PCHL; PPUL) to a microprocessor (7) and they represent the load on the transformer (T). One of the sense circuits (L 2 , R 8 , R 9 , C 3 ) comprises the second primary winding (L 2 ) and is used for measuring light loads. It controls, whether the second storage capacitor C 2  is to be charged and can in certain cases control the charge voltage. The second sense circuit (R 10 , R 11 , R 12 , T 4 , R 13 ) is connected to one terminal of the storage capacitors (C 1 , C 2 ) and is used for measuring heavy loads. It controls the time for a start of the discharging of the second storage capacitor and can also in certain cases control the charge voltage. In that way the supplied pulses will have a high voltage and a large energy content and a good safety is achieved.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electric fence energizer of thedischarge type, i.e. comprising a capacitor, which is charged to a highvoltage and is discharged to the primary winding of a transformer, thesecondary winding of the transformer providing a very high voltage tothe electric fence circuit. The energizer is in particular intended tobe operated by the mains supply, that is by the alternating voltage fromthe public electric energy distribution network.

2. Description of the Prior Art

Various requirements from the authorities restrict the electric voltagepulses which are allowed to be supplied to an electric fence. Theconventional requirements in Western Europe are thus that the maximumvoltage in each pulse is at most 10 kV over the output terminals of theelectric fence energizer, the largest electric current per pulse througha human being or through an animal is allowed to be 10 A, each electricpulse is not allowed to carry more energy than 5 joules, which can beprovided to a human being or an animal contacting the electric fence,the pulses are not allowed to come more frequently than one pulse persecond and the length of each pulse should be smaller than 1.5 ms andfinally that the total amount of charge in each pulse, which can beprovided to a human being or an animal contacting the fencing network,should be less than 2.5 millicoulombs. Naturally, all these requirementsexist in order to reduce the risk for damages to human beings andanimals which contact the electric fence network. However, in order thatan electric fence should efficiently limit or deter animals, the pulsesprovided from the electric fence should both have as large voltage aspossible and have as large energy as possible, within the limits imposedby the authorities.

An electric fence considered as an electric circuit, however, presentslarge variations depending on weather, earthing, and other factors whichinfluence the isolation of the fence wire in relation to the earth orthe ground. The resistance of the electric fence to ground can thus fordry weather and otherwise dry exterior conditions with good isolation bevery large compared to the condition which can be obtained, when forinstance a human being is in contact with the fence, when the resistancecan decrease to about 500 ohms. For extreme exterior conditions, inaddition, the resistance can decrease to still lower values. Theelectric fence circuit also comprises a capacitive part which can beimportant, when the resistance of the fence is large and which can causethat the circuit operates as a swinging circuit owing to the inductancein the transformer winding which supplies the high voltage pulses to thefence circuit. It can result in overswings in the voltage pulsegenerated on the fence side, which causes that the charge voltage forthe capacitor, from which the pulse is discharged, must be reduced inorder that the output pulses should not be too high. Then, without asuitable control, a reduced voltage will be obtained also in thosecases, when the fence circuit only has an insignificant capacitivecomponent compared to the fence resistance.

A possibility of obtaining an electric fence having a good efficiency isusing two transformers, one of which is used for providing high voltagepulses, when the electric fence has a good isolation to earth, alsoincluded in this isolation one or several human beings or one or severalanimals in contact with the fence, and another is used in the case wherethis isolation is not as good, such as for humid weather. In the lattercase, for circuit technical reasons, only smaller voltage pulses can besupplied but they will then instead be given a larger energy content.Alternatively a single transformer having two separate primary windingscan be used.

In the British Patent Specification No. 1 395 498 an embodiment of anelectric fence device is disclosed, see FIG. 1, having a second primarywinding 4. The voltage over this extra winding is used for control ofthe discharge process.

In the published German patent application No. 41 40 628 an averagevalue of a voltage pulse supplied to an electric fence is sensed andused for control of the charging of a storage capacitor. The averagevalue is sense directly on the output side of the fence energizer, thisarrangement being dangerous since high voltage may be applied to controlcircuits of the energizer.

In the published German patent application No. 39 04 993 an electricfence energizer is disclosed having separate primary windings of one ortwo transformers, these windings being associated with separatelyarranged energy storage capacitors and discharge circuits therefor. Bothcapacitors or only one thereof may be discharged depending on an averagevalue of the peak voltage pulses supplied to the fence. The peak valuesof the voltage pulses are sensed on the fence side of the energizer,this obviously being hazardous or dangerous as stated above.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an electric fence energizerwhich provides high voltage pulses having a large energy content andwhich has good security functions.

With the electric fence energizer according to the invention, the moredetailed characteristics of which appear from the appended claims, thisobject is achieved.

There are thus sense circuits in the energizer, these circuits alwayssensing voltages or other characteristics on the primary side of thestep-up transformer. No galvanic connections need to be made to thefence circuit on the secondary side.

The electric fence energizer is thus preferably operated by analternating voltage, for instance from the public electric energydistribution network, and it has generally two separate storagecapacitors, which are charged by a common charging circuit to a highvoltage. The storage capacitors are discharged through separate primarywindings in a transformer and the secondary winding of the transformeris in the conventional way connected to the electric fence. Thedischarge processes of the storage capacitors are controlled by separatedischarging circuits, so that for light loads--a light load means a highresistance in the fence circuit to earth--only one of the storagecapacitors is discharged, and for heavy loads--heavy loads are obtainedfor a small resistance in the fence circuit--also the other storagecapacitor is discharged starting a short time after the start of thedischarging of the first one and during the same discharge cycle. Sensecircuits provide signals to the charging circuits and dischargingcircuits as processed and evaluated by a microprocessor and thesesignals represent in various cases the load on the transformer, that isfrom the fence circuit. One sense circuit comprises the second primarywinding and is used for measurement of light loads. It can for verylight loads, when the capacitance in the fence circuit can be important,reduce the voltage, to which the storage capacitors are charged. Theother sense circuit is connected to a terminal of the storage capacitorsand is used for a measurement of heavy loads. It controls, whether thesecond storage capacitor should be discharged at all during a dischargecycle and in that case the time of the start of the discharge of thesecond storage capacitor. It can, in short circuit cases with, a verylow resistance in the fence circuit, also reduce the charge voltage forthe storage capacitors. The discharge of the first storage capacitor ismade, during a first short time period, through a circuit having alarger resistance than resistance which exists during the rest of thedischarge.

Thus, there is generally an electric fence energizer which preferably isoperated by an alternating voltage, in particular from the publicelectric energy distribution network. In the energizer there is a firststorage capacitor and a charging circuit connected to the alternatingvoltage and the first storage capacitor for charging the storagecapacitor to a high voltage. Further there is a first primary windingbelonging to a transformer, a secondary winding of the transformer beingconnected to the electric fence and the primary winding being connectedto the first storage capacitor. A discharge circuit is provided for thefirst storage capacitor and it is arranged to periodically discharge thefirst storage capacitor through its connected primary winding, forgenerating discharge pulses, which from the secondary winding of thetransformer are supplied to a connected electric fence.

In a first aspect, there is a sense circuit for sensing the load on thetransformer from a connected electric fence and for providing a signalrepresenting the load, the sense circuit comprising a second separateprimary winding of the transformer and a sense line connected to thatwinding. The sense circuit is arranged for sensing, during a dischargeof the first storage capacitor, at a selected time, that is at a timecontrolled or set by a controlling device such as a microprocessor, theinstantaneous magnitude of the voltage induced in the second primarywinding, this sensed magnitude being a measure or representing the loadon the transformer from a connected fencing network.

The sense circuit can be an extreme value sensing circuit connected tothe second primary winding and it then senses a maximum of the absolutevalue of voltage pulses induced in the second primary winding duringdischarges of the first storage capacitor. When the sensing times areset by a controller, the absolute value may be sensed at varying timesfrom the start of a discharge pulse. Thus the sense circuit can bearranged to sense, during successive discharges of the first storagecapacitor, the instantaneous magnitude of the voltage induced in thesecond primary winding at times, which are selected, so that the timeperiods from the start of the discharge of the first capacitor to thesensing time have different lengths, and then these sensed magnitudescan be evaluated for a determination of a maximum of the absolute valueof the voltage pulses induced in the second primary winding.

The charging circuit for the first capacitor can then be connected tothe sense circuit for controlling the voltage, to which the firststorage capacitor is charged by the charging circuit, depending on thevalue or values sensed by the sense circuit.

There may also be arranged a second storage capacitor, which also has acharging circuit connected to the alternating voltage and to the secondcapacitor for charging it to a high voltage, this charging circuitpreferably being common to both storage capacitors. This second storagecapacitor is then connected to the second primary winding of thetransformer. There is also a discharging circuit for the second storagecapacitor, which is arranged to discharge it through the secondaryprimary winding, for generating, in the same way as for the firststorage capacitor and the first primary winding, discharge pulses, whichare delivered by the secondary winding of the transformer to a connectedelectric fence.

The control device, generally a microprocessor, is connected to thesense circuit and to the discharging circuit for the second storagecapacitor and it is arranged for deciding, depending on the signal fromthe sense circuit, whether the discharging of the second storagecapacitor is or is not to be started, during each periodic discharge ofthe first capacitor, the discharge of the first capacitor always takingplace periodically, at evenly distributed time intervals.

The control device may naturally also be connected to the dischargingcircuit for the first storage capacitor and is then arranged to alwaysfirst start the discharging of the first storage capacitor and to startor not to start, at a time thereafter, while the discharging of thefirst storage capacitor is still in process, in parallel therewith, thedischarging of the second storage capacitor depending on the signal fromthe sense circuit representing the load, which is sensed by the sensecircuit during the time period from the start of the discharging of thefirst storage capacitor to the start of the discharging of the secondcapacitor, the sensing and the start of the discharge of the secondcapacitor always being made during the same discharge process or cyclefor the first storage capacitor.

Thus generally, the sense circuit senses the load on the transformerfrom the fence and provides a signal representing the load and thereforit comprises an extreme value sensing circuit for sensing the maximum ofthe absolute value of a voltage pulse, which is obtained at adischarging of the first storage capacitor. It provides a signalrepresenting the sensed maximum to the charging circuit for the firststorage capacitor and this charging circuit is arranged to control avoltage, to which the first storage capacitor is charged thereby,depending on the signal representing the maximum sensed by the sensecircuit.

In another aspect, the sense circuit comprises a conductive lineconnected to a first terminal of the first storage capacitor and thereis in the energizer a discriminating circuit connected to this conductorfor sensing the time, at which, during a discharging cycle or process ofthe first storage capacitor, the voltage over the first storagecapacitor has decreased to a predetermined value.

The sense circuit may then comprise a transistor, the base of which,through a voltage divider or potentiometer circuit, is connected to afirst terminal of the first storage capacitor, the second terminal orelectrode of the capacitor being connected to ground.

The charging circuit for the first storage capacitor may then bearranged to reduce a voltage, to which the first storage capacitor ischarged by the charging circuit, when the sense circuit senses a veryheavy load, in particular a short circuit, in an electric fenceconnected to the transformer.

In this aspect, also a second storage capacitor may be arranged having acharging circuit connected to the alternating voltage for charging thesecond storage capacitor to a high voltage. A second primary windingwhich is different from the first primary winding, belongs to atransformer, which preferably is the same as the transformer associatedwith the first primary winding and the first storage capacitor, and asecondary winding of the transformer is connected to the electric fenceand the second primary winding is connected to the second storagecapacitor. There is a discharging circuit for the second storagecapacitor, which is arranged to discharge it through the second primarywinding, for generating, in the same way as for the first storagecapacitor and the first primary winding, discharge pulses, which aredelivered by the secondary winding of the transformer to a connectedelectric fence.

The discharging circuit for the first storage capacitor at eachdischarge period can then be arranged to first start the discharging ofthe first storage capacitor and the discharging circuit for the secondstorage capacitor is arranged to then start, during this dischargingprocess or cycle of the first storage capacitor, at a time depending onthe load sensed by the sense circuit, the discharging of the secondstorage capacitor.

In still another aspect, there are first and second storage capacitorsand charging circuits connected to the alternating voltage and to thefirst and the second storage capacitor respectively for charging thefirst and the second storage capacitor respectively to a high voltage,the charging circuits preferably being the same circuit used for the twocapacitors. There are separate, first and second primary windingsbelonging to transformers, generally the same one, and secondarywindings of the transformers are connected to the fence. The first andsecond primary windings are then as above connected to the first and thesecond capacitor respectively. Discharging circuits for the first andsecond storage capacitor are arranged to periodically discharge thefirst and the second storage capacitor respectively through itsassociated primary winding, for generating discharge pulses, which fromthe secondary winding of the respective transformer are supplied to aconnected electric fence.

A sense circuit is as above arranged for sensing the load on thetransformers from a connected electric fence and for providing a signalrepresenting the sensed load. The discharging circuit for the firststorage capacitor is arranged, at each discharge period, to first startthe discharging of the first storage capacitor and the dischargingcircuit for the second storage capacitor is arranged to start, duringthis discharging, at a time depending on the load sensed by the sensecircuit, the discharging of the second storage capacitor.

BRIEF DESCRIPTION OF THE DRAWING

An embodiment of the invention will now be described with reference tothe accompanying drawing, in which

FIG. 1 shows a circuit diagram of a mains operated electric fenceenergizer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An electric circuit for an electric fence energizer is in its essentialparts shown by the circuit diagram of FIG. 1. An alternating voltage,e.g. from the public electric energy distribution network, is suppliedbetween terminals 1 and 3 to a charging circuit 5. A microprocessor 7controls the charging circuit 5 for charging two, equally large storagecapacitors C₁ and C₂, which have a large capacitance and are connectedin parallel with their first terminals or plates to the charging circuit5, and thus are charged by the charging circuit 5 to the same voltage,the charge voltage. The charge voltage is maximally about 630 V but canbe given, by the microprocessor 7 when required, a lower value. The twostorage capacitors C₁ and C₂ have both their second terminals or platesconnected to electronics or signal ground and the first terminals areeach one connected to a separate primary winding L₁ and L₂ respectivelyof a transformer T, which is provided with a single secondary windingL₃. The secondary winding L₃ supplies high voltage pulses to the fencecircuit, not shown, which is connected between terminals 9 and 11 of thesecondary winding. The fence circuit is described in more detail in oursimultaneously filed International application PCT/SE94/01268 filed Dec.29, 1994 having the title "Defective earth testing for an electric fenceenergizer", which is incorporated herein by reference.

The other end of the first primary winding L₁ is through a resistor R₁connected to the positive terminal or electrode of a first thyristorTy₁, the negative electrode of which is connected to electronics ground.The electronic circuits have a made-up or artificial ground connection,which has a potential corresponding to either one of the poles of thesupplied mains voltage, i.e. equal to the potential of a phase, theground or the neutral conductor. The gate electrode of the thyristor Ty₁is controlled by means of a signal TY1 from the microprocessor 7, whichis provided through a resistor R₂ to the base of a transistor T₁, theemitter of which is connected to the gate electrode of the firstthyristor Ty₁. The collector of the transistor T₁ is through a collectorresistor R₃ connected to a positive supply voltage E₁ of e.g. 12 V, thissupply voltage being a constant voltage in relation to signal ground.The first primary winding L₁ of the transformer T is in parallelherewith connected to the positive electrode of a second thyristor Ty₂,but without any resistor in the connection line. The negative electrodeof the thyristor Ty₁ is connected to the electronics ground. Thisthyristor Ty₂ is controlled in a similar way as the thyristor Ty₂ bymeans of a signal TY2 from the microprocessor 7, which is deliveredthrough a base resistor R₄ to the base of a transistor T₂, the emitterof which is directly connected to the gate electrode of the thyristor.The collector of the transistor T₂ is through a resistor R₅ connected tothe positive supply voltage E₁.

Also the second primary winding L₂ of the transformer T has its secondterminal connected to the positive electrode of a thyristor, a thirdthyristor Ty₃, and the negative electrode of this thyristor Ty₃ is alsoconnected to electronics ground like the two other thyristors Ty₁ andTy₂. Also this third thyristor Ty₃ is controlled in the correspondingway by a signal TY3 from the microprocessor 7, which through a baseresistor R₆ is delivered to the base of a transistor T₃, the emitter ofwhich is connected to the gate electrode of the third thyristor Ty₃. Thecollector of the transistor T₃ is through a resistor R₇ connected to thepositive supply voltage E₁.

The load in the shape of the fence circuit connected to the secondarywinding L₃ of the transformer T is evaluated or measured in twodifferent ways. To be used for light loads and high output voltages avoltage divider circuit is arranged in the shape of resistors R₈ and R₉,which is connected between the terminals of the second primary windingL₂ of the transformer T. At the centre point of the voltage divider,between its resistors R₈ and R₉, a signal is drawn in the shape of avoltage which is delivered to an input port PPUL of the microprocessor7. The centre point of the voltage divider is also through a capacitorC₃ connected to signal ground. The signal PPUL is a high positivevoltage for the illustrated polarities, when the load from the fencecircuit is heavy.

The measurement is performed more accurately in such a way that themicroprocessor 7 at a selected time sets its input PPUL to the potentialof the signal ground conductor, whereby the capacitor C₃ is completelydischarged. Then the input port PPUL is displaced to a state having ahigh resistance, whereby the voltage supplied through the voltagedivider R₈, R₉ charges the capacitor C₃. The voltage over this capacitorC₃ increases and finally achieves a voltage value corresponding to alogical high level on the input PPUL of the microprocessor 7. The lengthof the time period, which has elapsed during charging the capacitor C₃to this level, is measured by the microprocessor 7 and forms a measureof the voltage over the second primary winding L₂. The capacitance ofthe capacitor C₃ is chosen to have such a small value that the wholemeasuring procedure is performed during a short time, during which thevoltage over the second primary winding L₂ changes little.

A second evaluation of the load to be used, when the load is heavy (asmall resistance in the fence circuit, i.e. between the terminals 9 and11) and thus a low output voltage is delivered from the secondarywinding of the transformer T, is given by a signal obtained on an inputport PCHL of the microprocessor 7. The first terminals of the storagecapacitors C₁ and C₂ are through a resistor R₁₀ connected to thecharging circuit 5 and to this connection point between the chargingcircuit 5 and the resistor R₁₀ also the base of a transistor T₄ isconnected through a voltage divider circuit comprising a resistor R₁₁connected to the aforementioned point and a resistor R₁₂ which has itsone terminal connected to signal ground. The charge voltage of thestorage capacitor C₁ is caused to proportionally, by means of thevoltage divider, drive this transistor T₄. The transistor T₄ is,different from the other transistors, of PNP-type and has its emitterconnected to a positive stable and constant supply voltage E₂, e.g. thesupply voltage of 5 V, which is conventionally used for driving themicroprocessor 7, the constant level being taken in relation to thesignal ground connection, and has its collector connected through aresistor R₁₂ to electronics ground. The measurement signal is deliveredto the input port PCHL of the microprocessor from the collector of thetransistor T₄.

The measurement process is also here, as described more accurately, atime measurement. The base of the transistor T₄ is, at the start of thedischarge of the storage capacitors C₁, C₂, at a high potential and thusthe current through the transistor T₄ is blocked. When the dischargeprocess continues, however, the voltage over the voltage divider R₁₁,R₁₂ decreases to finally give a so low potential at the centre point ofthe voltage divider circuit, at the base of the transistor T₄, that thetransistor T₄ starts to conduct. The potential at the collector of thetransistor T₄ then increases from an initially low value to a valuecorresponding to a high logical level at the input ports of themicroprocessor 7, which can correspond to 100 to 300 V over the primarywindings L₁, L₂ of the transformer T. This time can then be sensed bythe microprocessor 7 and the time length from the start of thedischarging of the first storage capacitor C₁ to this time is then ameasure of the resistive lead between the output terminals 9, 11 of thetransformer T. The choice of the voltage, at which a transition occurs,i.e. when the transistor T₄ starts to conduct, is important in thosecases where the fence lead has a significant capacitive component. If atoo high change voltage is chosen--by selecting suitable magnitudes ofe.g. the resisters R₁₀, R₁₁ and R₁₂ --a lead having a capacitive partwill result in a shorter time, before the transistor T₄ changes over,and thus be detected as a heavier lead. This measurement functions aslong as the lead of the fence is so heavy that the capacitor has time tobe discharged, before the iron core of the transformer T will bemagnetically saturated.

The discharge of the storage capacitors C₁ and C₂ occurs principally insuch a way that first the discharging of the storage capacitor C₁ isstarted through the first primary winding which is provided with asmaller number of winding turns than the second primary winding L₂.Hereby a high output voltage is induced having an order of magnitude ofapproximately those 10 kV which are allowed in the fence circuit. Thisis valid when the fence circuit is a small load on the transformer.After some discharging, more particularly after a certain controllabletime period after the start of the discharging of this first storagecapacitor C₁, the discharging can, if required, be started from thesecond storage capacitor C₂ over the second primary winding L₁, wherebythe discharge pulse is reinforced and gains further energy. In orderthat the discharge current from the second capacitor then will not passthrough the first primary winding L₁, the connection of the secondstorage capacitor C₂ is made through a diode D₁ to the charging circuit5.

The load is determined by the microprocessor 7 and its value isevaluated by the microprocessor in order to decide whether at all thesecond storage capacitor C₂ is to be connected and in that case in orderto determine a suitable time for the start of the discharge of thesecond storage capacitor C₂.

The discharge of the storage capacitors C₁ and C₂ is determined by meansof the thyristors Ty₁, Ty₂ and Ty₃. These are, at the charging processof the storage capacitors C₁, C₂, blocked and are caused to conduct bymeans of the control signals TY1, TY2, and TY3 respectively, obtainedfrom the microprocessor 7. In the discharging of the first storagecapacitor C₁, which gives the high voltage on the secondary side of thetransformer T, first the thyristor Ty₁ is ignited. Then the firststorage capacitor C₁ is discharged through the first primary winding L₁in series with the resistor R₁. This discharging will hereby be a littleattenuated and reduces the tendency to overvoltages or overswings of thegenerated voltage on the secondary winding L₃ of the transformer T,which can occur, when the load in the shape of the fence circuit,connected between the terminals 9, 11, has a capacitive component. Then,after a small, predetermined time period the second thyristor Ty₂ isignited by means of the signal TY2. At this time, when the voltage overthe first storage capacitor C₁ has decreased a little, the discharge ismade through the first primary winding L₁ directly through the thyristorTy₂.

Finally, also the second storage capacitor C₂ can be caused to bedischarged, by causing the thyristor Ty₃ to conduct as controlled by thesignal TY3 and then the discharge of the second storage capacitor C₂ ismade through the second primary winding L₂ directly through thethyristor Ty₃.

During that time period, when only the first storage capacitor C₂ isdischarged, also a voltage is induced in the second primary winding L₂and this signal is evaluated at different times by means of the signal,which is provided to the processor 7 on its input terminal PPUL.

During the discharge cycles, when also the second storage capacitor C₂is discharged through the transformer T, it is performed, by means ofthis signal on the input terminal PPUL, an instantaneous measurement ofthe load between the terminals 9, 11, on the secondary side of thetransformer T during exactly this discharge pulse from the first storagecapacitor C₁ before the start of the discharge of the second capacitorC₂. The result of this measurement is used by the microprocessor 7 inorder to control that the delivered voltage is not too high and thusthat the load has not decreased or become lighter. If the voltage shouldbe too high, the discharge of the second storage capacitor C₂ is notstarted at all.

The signal on the input PPUL of the microprocessor 7 is also used forproviding an accurate measurement of the load for high output voltagesand light loads. It can during longer time periods, when the dischargeof the second storage capacitor C₂ does not need to be started owing tothe light load, be evaluated during several successive discharge cyclesfor the first storage capacitor C₁. From this signal a value of themaximum voltage of the discharge pulse can be derived, i.e. generallythe maximum of the absolute value. The discharge pulse will, as has beenmentioned above, have different appearances depending on the load andamong other things on the capacitive component thereof. The measurementof the maximum voltage is made in such a way, that the voltage of thedischarge pulse is measured at different times, as considered from thestart of the discharge, during successive discharge pulses. The largestvalue determined in that way is then the desired maximum value. When thedetermined maximum value is too high, the microprocessor 7 can controlthe charging circuit 5 for the storage capacitors C₁ and C₂ in such away, that they instead of being charged to the normal 630 V instead arecharged to for instance about 500 V. Hereby one can achieve that theenergizer gives output pulses lower than the limit values of theauthorities, but that at the same time output pulses are obtained havinga voltage, which is as high as possible.

For heavy loads and thus small output voltages the microprocessor 7 usesthe signal on the input terminal PCHL, which then gives an accuratedetermination of the load. Then normally also, the second storagecapacitor C₂ is used to add more energy to the voltage pulse on theoutput side of the transformer T and the time for connecting the secondstorage capacitor is determined by the microprocessor 7 by means of thevalue determined from the signal on the input terminal PCHL.

The load is thus measured during each discharge cycle by means of thesignal on the input terminal PCHL and in particular, the valuedetermined from this signal is used for deciding whether heavy loads,for instance smaller than 20 ohms such as for a short circuit, exist inthe fence circuit. In that case an overheating can occur in the device,in particular in the windings of the transformer T, and in that case themicroprocessor 7 decides, in accordance with a control scheme or controlprogram entered therein, that the charge voltage for the storagecapacitors C₁ and C₂ is to be reduced to some suitable value.

We claim:
 1. An electric fence energizer comprising:a first storagecapacitor: a charging circuit connected to an alternating voltage andthe first storage capacitor for charging the storage capacitor to a highvoltage; a transformer being subject to a load and having a firstprimary winding and a secondary winding, the primary winding beingconnected to the first storage capacitor and the secondary winding beingconnectable to an electric fence; a discharge circuit for the firststorage capacitor which is arranged to periodically discharge the firststorage capacitor through its connected primary winding for generatingdischarge pulses supplied from the secondary winding of the transformerto a connected electric fence; and a sense circuit for sensing the loadon the transformer from a connected electric fence and for providing asignal representing the load, the transformer having a second separateprimary winding connected to the sense circuit via a sense line, thesense circuit including means for sensing the instantaneous magnitude ofthe voltage induced in the second primary winding at a selected timeduring a discharge of the first storage capacitor.
 2. An electric fenceenergizer according to claim 1, wherein the second primary winding hasmore winding turns than the first primary winding.
 3. An electric fenceenergizer according to claim 1, wherein the sense circuit includes anextreme value sensing circuit connected to the second primary windingfor sensing a maximum of the absolute value of voltage pulses induced inthe second primary winding during discharges of the first storagecapacitor.
 4. An electric fence energizer according to claim 3, whereinthe sense circuit includes means for sensing, during successivedischarges of the first storage capacitor, the instantaneous magnitudeof the voltage induced in the second primary winding at times, which areselected, so that the time periods form the start of the discharge ofthe first capacitor to the sensing time have different lengths, andmeans for evaluating these sensed magnitudes for a determination of amaximum of the absolute value of the voltage pulses induced in thesecond primary winding.
 5. An electric fence energizer according toclaim 3, wherein the charging circuit for the first capacitor isconnected to the sense circuit and includes means for controlling avoltage, to which the first storage capacitor is charged by the chargingcircuit, depending on the maximum sensed by the sense circuit.
 6. Anelectric fence energizer according to claim 1, further comprising:asecond storage capacitor; a charging circuit connected to thealternating voltage and the second storage capacitor for charging it toa high voltage, the second storage capacitor being connected to thesecond primary winding of the transformer; and a discharging circuit forthe second storage capacitor for discharging the second storagecapacitor through the second primary winding for generating, in the sameway as the first storage capacitor and the first primary winding,discharge pulses which are delivered by the secondary winding of thetransformer to a connected electric fence.
 7. An electric fenceenergizer according to claim 6, further comprising a control deviceconnected to the sense circuit and discharging circuit for the secondstorage capacitor for deciding whether the discharging of the secondstorage capacitor is or is not to be started during each periodicdischarge of the first capacitor depending on the signal from the sensecircuit.
 8. An electric fence energizer according to claim 7, whereinthe control device is also connected to the discharging circuit for thefirst storage capacitor and is arranged to always first start thedischarging of the first storage capacitor and to start or not to start,at a time thereafter, while the discharging of the first storagecapacitor is still in progress, in parallel the discharging of thesecond storage capacitor depending on the signal from the sense circuitrepresenting the load which is sensed by the sense circuit during thetime period from the start of the discharging of the first storagecapacitor to the start of the discharging of the second capacitor.
 9. Anelectric fence energizer according to claim 1, wherein the sense circuitincludes an electric storage means charged by the induced voltage in thesecond primary winding.
 10. An electric fence energizer according toclaim 9, wherein the sense circuit comprises time measurement means formeasuring the length of the time period for charging the storage meansfrom the second primary winding.
 11. An electric fence energizercomprising:a first storage capacitor; a charging circuit connected to analternating voltage and the first storage capacitor for charging thestorage capacitor to a high voltage; a transformer having a firstprimary winding and a secondary winding, the primary winding beingconnected to the first storage capacitor and the secondary winding beingconnectable to an electric fence; a discharging circuit for the firststorage capacitor for periodically discharging the first storagecapacitor through its connected primary winding for generating dischargepulses which are transferred from the secondary winding of thetransformer to a connected electric fence; and a sense circuit forsensing the load on the transformer from a connected electric fence andfor providing a signal representing the load, the sense circuitincluding an extreme value sensing circuit for sensing the maximum ofthe absolute value of a voltage pulse which is obtained at a dischargingof the first storage capacitor, and for providing a signal representingthe sensed maximum, the charging circuit for the first storage capacitorbeing connected to the sense circuit and being arranged to control avoltage to which the first storage capacitor is charged by the chargingcircuit pending on the signal representing the maximum sense by thesense circuit.
 12. An electric fence energizer according to claim 11,wherein the sense circuit includes means for sensing, during successivedischargings of the first storage capacitor, the instantaneous magnitudeof a voltage pulse which is obtained by discharging the first storagecapacitor at selected times such that they occur at differently largetime intervals from the start of the discharging of the first storagecapacitor during each discharge cycle thereof, and for comparing andevaluating the sense magnitudes for determination of a maximum of theabsolute value of the voltage pulses obtained in the dischargings of thefirst storage capacitor.
 13. An electric fence comprising:a firststorage capacitor; a charging circuit connecting to an alternatingvoltage and the first storage capacitor for charging the first storagecapacitor to a high voltage; a transformer having a first primarywinding and a secondary winding, the first primary winding beingconnected to the first storage capacitor and the secondary winding beingconnectable to an electric fence; a discharging circuit for the firststorage capacitor including means for periodically discharging the firststorage capacitor through the first primary winding of the transformerfor generating discharged pulses which are supplied to a connectedelectric fence from the secondary winding of the transformer; and asense circuit for sensing the load on the transformer from a connectedelectric fence and for providing a signal representing the sense load,the sense circuit including a conductive line connected to a firstterminal of the first storage capacitor and a discriminating circuitconnected to said conductive line for sensing the time at which thevoltage across the first storage capacitor has decreased to apredetermined value during a discharge of the first storage capacitorand a transistor, the base of which is connected to the first terminalof the first storage capacitor through a voltage divider.
 14. Anelectric fence energizer according to claim 13, wherein the chargingcircuit for the first storage capacitor includes means for reducing avoltage by which the first storage capacitor is charged by the chargingcircuit when the sense circuit senses a short circuit in an electricfence connected to a transformer.
 15. An electric fence energizeraccording to claim 13, further comprising:a second storage capacitor; acharging circuit connected to the alternating voltage and the secondstorage capacitor for charging the second storage capacitor to a highvoltage; a second primary winding of the transformer which is differentfrom the first primary winding and which is connected to the secondstorage capacitor; and a discharging circuit for the second storagecapacitor for discharging said second storage capacitor through thesecond primary winding for generating, in the same way as for the firststorage capacitor and the first primary winding, discharge pulses whichare delivered by the secondary winding of the transformer to a connectedelectric fence, the discharging circuit for the first storage capacitorat each discharged period including means for first starting thedischarge of the first storage capacitor, the discharging circuit forthe second storage capacitor including means for then starting thedischarge of the second storage capacitor during this discharge of thefirst storage capacitor at a time depending upon a load sensed by thesense circuit.